What is interstellar dust?
It is a question that has been asked several times and, from what I've been able to ascertain, many people have many different ideas as to what these little dust particles are. In Earth, what we consider to be dust isn't exactly what we find out in space, as I will explain.
The dust that you find in your home is a bit different to the dust that you find outside. The dust inside the house can be made of:
-Dirt stuff that comes from outside (obviously)!
-Dead skin and hair from humans,pets, and pests.
-Dead dust mites and their... matter (try not to think about that).
-Mold, bacteria, spores, decaying matter.
-Textile fibers, such as carpet, blinds, and clothes.
Dust outside of the home however is made up of two main things.
-Silicates, such as rock and sand (everything that the Earth is actually made of).
-EVERYTHING else that exists on this planet.
Really, there isn't a huge different between the dust found inside and the dust found outside, it's just silicates and organic matter. So you might be asking yourself what the actual difference is-- the difference is the concentrations of the different materials: silicates vs organic material. Dust outside typically contains more silicates and inside contains more organic material and fibers.
I guess I can't really go ahead and say that the dust in interstellar space is made of decaying organic material, it would be a weird dream come true for SETI ...but it wouldn't be close to the truth. The average dust particle size on Earth are quite large, most of the dust that we see are between 1-100+ microns in size. In the "further reading" section, there is a long list of particles and their sizes that are found on the Earth.
So this dust stuff that we find out in the rest of the universe... What is it? Actually, it's quite similar to the atmospheric dust particles that we find on the Earth, but there is a slight difference. Interstellar dust particles typically have a core composed of either silicates or metals, sometimes a little of both. The core is then surrounded by ice because, as you can imagine, it is pretty damn cold out there in the deep blackness of the universe. Interstellar dust is typically on average about 0.2 microns in size, some can be as large as 50 microns and some as small as just a dozen or so atoms!
This dust is produced in several different locations in the universe. It is formed in the cooler outer layers of Red Giant stars, and it is subsequently blown off by the solar winds or after the star dies and leaves behind a planetary nebula (and a white dwarf). They can also be formed in the expanding shell of gas left over from a supernova remnant if given some time after the supernova (time allows the expanding shell of gas to move far enough away from the neutron star--in the case of the Crab Nebula-- so it becomes cool enough, while still be dense enough, for dust grains to form).
It is often very obvious where large concentrations of interstellar dust reside-- just look for the black colourless parts of an image. One of the most famous image is that depicted above, the Horsehead Nebula. Generally, when we discuss a nebula, we're actually referring to a nice colour image of the emission and reflection of Hydrogen (mostly) and Oxygen. With the Horsehead Nebula though, the main object of that image is non emitting and boring dust, but the image that it depicts is just truly magical and magnificent.
For further reading:
"Particle Sizes on Earth"